There has conventionally been proposed an rotary eddy current testing probe device for detecting flaws in all directions in an electric conductor such as a metal by rotating and moving a rotating disc to which a plurality of eddy current testing probes are attached (for example, refer to JP2007-248169A).
In this description, an eddy current testing probe is referred to as an eddy current probe, and a device including a rotating disc to which a plurality of eddy current testing probes are attached is referred to as a rotary eddy current testing probe device. The rotary eddy current testing probe device is abbreviated to a rotary eddy current probe device as appropriate.
A coil axis means an axis that is the center of spiral winding of windings constituting a coil, and a coil plane means a plane perpendicular to the coil axis.
A conventional rotary eddy current probe device including four eddy current probes attached to a rotating disc will be described with reference to FIGS. 7A1, 7A2 and 7B.
FIG. 7A1 is a plan view of the rotary eddy current probe device and an object being inspected, FIG. 7A2 is a sectional view taken along the line X4-X4 of FIG. 7A1, and FIG. 7B is a graph showing the amplitude characteristic of a flaw signal detected by the rotary eddy current probe device.
A rotary eddy current probe device RP2 has four Θ-shaped eddy current probes P31 to P34 and a rotating disc 211. The eddy current probes P31 to P34 are embedded in the rotating disc 211 by molding, and are arranged so as to face the inspection surface of an object being inspected 22 for which the presence of a flaw is inspected. The Θ-shaped eddy current probe has an exciting coil for exciting an eddy current in the object being inspected and a detector coil disposed on the inside of the exciting coil to detect the eddy current excited in the object being inspected, and both the coils are arranged so that the respective coil planes are perpendicular to each other. That is, the coil plane of the exciting coil is parallel with the rotation plane of the rotating disc 211, and the coil plane of the detector coil is perpendicular to the rotation plane of the rotating disc 211. The rotating disc 211 is rotated by the rotation of a rotating shaft 212, and the rotating shaft 212 is rotated by a motor (not shown).
The four eddy current probes P31 to P34 are arranged at approximately equal intervals (intervals of 90 degrees) in order in the circumferential direction around the rotation center Ds2. Of the four eddy current probes P31 to P34, the eddy current probes P31 and P33 are located on opposite sides with respect to the rotation center Ds2, and the eddy current probes P32 and P34 are also located on opposite sides with respect to the rotation center Ds2. That is, the four eddy current probes P31 to P34 are composed of two sets: a set of the eddy current probes P31 and P33 and a set of the eddy current probes P32 and P34. One set of the detector coils and the other set of the detector coils are arranged so that the coil planes are perpendicular to each other and the coil axes are also perpendicular to each other.
The following is an explanation of a flaw signal detected when the object being inspected 22 shown in FIG. 7A1 is inspected by using the rotary eddy current probe device RP2.
On the inspection surface of the object being inspected 22, a flaw F31 elongated in parallel with the movement direction (X5 direction) of the rotary eddy current probe device RP2, a flaw F32 slantwise intersecting with the movement direction, and a flaw F33 intersecting at right angles with the movement direction are formed. All of the flaws F31, F32 and F33 each have a length (length of the long side) of 150 mm, a width (width of the short side) of 0.5 mm, and a depth of 0.3 mm.
The rotary eddy current probe device RP2 excites an eddy current in the object being inspected by using the exciting coil, detects the eddy current excited in the object being inspected by using the detector coil, and detects a flaw on the basis of a signal detected by the detector coil. The four detector coils are cumulatively connected. The signal detected by the detector coil is referred to as a flaw signal.
When the rotary eddy current probe device RP2 is moved in the X5 direction along the inspection surface of the object being inspected 22 while being rotated, a flaw signal caused by the flaws F31 and F32 can be detected, but a flaw signal caused by the flaw F33 cannot be detected sufficiently. That is, the flaw signal caused by the flaw F33 is as shown in FIG. 7B. In FIG. 7B, the ordinate represents the detection ratio that is the ratio of the amplitude of the signal of a detected flaw to the maximum amplitude, and the abscissa represents the movement width in the X5 direction of the rotary eddy current probe device RP2. The zero point on the abscissa corresponds to a position at the time when the rotation center Ds2 of the rotary eddy current probe device RP2 moves to just above the flaw F33. The rectangle drawn by a broken line in FIG. 7B indicates the range in which the detection ratio of flaw signal is −3 dB or more in front and rear of the flaw F33 and the range of the movement width of rotary eddy current probe device capable of detecting the flaw. The detection ratio of −3 dB is a signal detection ratio that is generally thought to be effective for flaw detection, and when the detection ratio is −3 dB or more, it is judged that a flaw is present.
In the case of FIG. 7B, despite the presence of flaw, the detection ratio of flaw signal is less than −3 dB at two locations indicated by arrows in the figure, which reveals the presence of a region in which the flaw signal is difficult to detect.